I had enormous stability problems with chromium 36 series ("aw snaps" every 10 minutes, even on pages that had successfully loaded only moments before). Since I updated to the latest 37 series (currently 37.0.2062.20), they have completely disappeared.

First, manually run eix-update. If you just sync'ed portage, it's there. But it's keyworded as arch, so you need to add it to your package.keyword file in order to emerge it. So, edit /etc/portage/package.keyword and add
www-client/chromium